Various methods are available to mechanically scan an optical beam and an associated receiver field-of-view through a region with the distance measurement based on the calculated round trip delay between the emission of a light pulse in the direction of an object and the subsequent reception of the received signal. The common approach for implementation of the scanning function is the use a mechanical steering mirror driven by an actuator to steer the beam and receiver field of view through the desired region. An alternative approach is to mount the transmitter and receiver on a moving platform such as a multiple-axis gimbals or rotary platform to accomplish this function. The transmitter and receiver can be mounted coaxially or in a parallel co-bore sited configuration depending on the requirements of the system.
The primary limitation of mechanically scanning the field of view using a steering mirror is a complex trade-off between power consumption, weight and size of the scanner, efficient use of the optical beam power and stray light rejection. The physical size of the scanning mirror needs to be sufficient to allow the passing of the transmit beam and reflected optical signal to be collected by the receiver lens. If the mirror scans a raster pattern over a rectangular field-of-view, the requirement to rapidly accelerate and decelerate the mirror increases power consumption and results in the inefficient use of the scanning time. To minimize power consumption, a fixed velocity rotational scanner can be used. An external, multiple-facet, rotating polygon mirror scanner can produce a rapid scanning pattern, but is limited to small transmit and receive apertures.
An alternative approach rotates a mirror at a 45-degree orientation to the beam about the center-axis of the transmitter and receiver to produce 360-degree coverage in a plane perpendicular to the rotational axis. This configuration allows the scanning of a larger receiver and transmit beam at the expense of a potentially undesirable by-product, the rotation of the relative position of the transmitted beam and the receiver field. When the exiting beam and receive field needs to pass through the curved optic of a cylindrical window or dome the relationship between the transmitted and received fields rotates, resulting in changing optical distortion and the potential for large amount of transmitter beam scattering into the receiver.
Published U.S. patent application US 2010/0020306A1 to David Hall describes a time-of-flight based 3-D LIDAR sensor using a rotating linear array of photo detection fields-of-view with associated laser elements for each pixel. The multiple receive signals are processed to estimate the time-of-arrival in each location. Receiver signal processing captures the received signal from individual channels allowing the subsequent extraction of the time-of-arrival using threshold detection. The goal of this configuration was to maximize the update rate number for a high number of resolution elements. The penalty paid for this design goal is high system complexity and associated cost.
U.S. Pat. No. 6,137,566 to Leonard et al. discloses a method based on individual processing channels for each detection region within a photodiode array. Because of the large number of parallel processing channels, the implementation of the pre-amplification, low resolution A/D and the need for GHz frame word rate dictates a high hardware complexity, limiting the approach to high performance military applications. This method is also inefficient when combined with low-duty cycle pulsed laser sources since the effective utilization of the receiver occurs for short durations relative to the pulse of the system.
A variety of CCD based detection methods have been disclosed to minimize the cost and complexity of detection. U.S. Pat. No. 6,906,793 to Bamji et al. illustrates a representative CCD based system consisting of the use of a modulated LED or laser source and a CCD detector array incorporating on-chip circuitry to extract the phase of the incoming signal. These CCD systems, in theory, provide virtually ideal signal integration characteristics at the expense of inefficient utilization of chip real estate due to the presence of phase mixing circuitry, the charge storage well and output shift registers. Unfortunately this method can only provide noiseless integration when the noise contribution of the background and thermal leakage currents are negligible relative to the signal shot noise. This occurs under very dark background conditions that are not achievable under outdoor and bright lighting indoors due to limitations in obtaining a sufficiently narrow optical notch filter spectral width. The optical notch filter's band pass characteristic needs to be matched to the spectral width and drift over temperature of the LED or laser source which can dictate a relatively broad band pass width. Narrow pass optical filters become inefficient and prohibitively expensive as the spectral width is reduced.
Methods and apparatus are needed that offer the simplicity and low cost potential of CCD 3-D image sensors, with the performance advantage of direct digitization of the receiver signal and consistent performance under bright background conditions.